In this letter a circular polarization microstrip antenna with switchable polarization is proposed. The switchable polarization sense characteristic is realized via a shunt connected varactor tuning diode. The appropriate capacitance of the diode at the reverse bias voltage can alter two circular polarizations, as the tuning diode, which is located near the rectangular slot in the circular patch, is utilized in a perturbation element. The switchable polarization is analyzed using the equivalent circuit model representing the resonances of each orthogonal mode. Simulation, calculated, and measured results agree well.

In this letter, compact loop resonator type circular polarization (CP) antennas with a square ring and an X-shaped meander loop are presented. Both antennas are fed to a microstrip line with electromagnetic coupling. By adjusting the gap and length of a coupled microstrip line, the magnitude and phase conditions of two orthogonal modes for CP can be determined. The proposed antennas show good axial ratios and also good agreements between experimented and simulated results.